1. Field of the Invention
The present invention relates to an adapter used in combination with a power amplifier for driving a loudspeaker (hereinafter referred to simply as a "speaker") in order to change transfer characteristics of the power amplifier and to drive the speaker. An adapter of this type is suitably used for effecting a negative impedance driving on a speaker when a commercially available power amplifier is used.
2. Description of the Prior Art
As a conventional driver for driving a speaker unit assembled in a speaker system, a power amplifier whose output impedance is substantially zero is generally used.
A conventional speaker system is arranged to exhibit optimal acoustic output characteristics when it is constant-voltage driven by such a power amplifier whose output impedance is substantially zero.
FIG. 7 is a sectional view of a conventional closed type speaker system. As shown in the Figure, a hole is formed in the front surface of a closed cabinet 1, and a dynamic speaker unit 3 having a diaphragm 2 and a driver is mounted over the hole.
A lowest resonance frequency f.sub.oc and a Q value Q.sub.oc at the frequency f.sub.oc of the closed type speaker system are respectively given by: EQU f.sub.oc =f.sub.o (1+S.sub.c /S.sub.o).sup.1/2 ( 1) EQU Q.sub.oc =Q.sub.o (1+S.sub.c /S.sub.o).sup.1/2 ( 2)
where f.sub.o and Q.sub.o are respectively the lowest resonance frequency and Q value of the dynamic speaker unit 3, i.e., those when the speaker unit 3 is attached to an infinite plane baffle. S.sub.o is the equivalent stiffness of a vibration system, and S.sub.c is the equivalent stiffness of the cabinet 1.
In the closed type speaker system, the lowest resonance frequency f.sub.oc serves as a standard of a bass sound reproduction limit of a uniform reproduction range, i.e., a lowest reproduction frequency. The Q value Q.sub.oc relate to a reproduction characteristic curve around the resonance frequency f.sub.oc. If the Q value Q.sub.oc is too large, the characteristic curve becomes too sharp around f.sub.oc. If the Q value Q.sub.oc is too small, the characteristic curve becomes too moderate. In either case, the flatness of the reproduction frequency characteristics is impaired. The Q value Q.sub.oc is normally set to be about 0.8 to 1.
FIG. 8 is a sectional view showing an arrangement of a conventional phase-inversion type (bass-reflex type) speaker system. In the speaker system shown in the Figure, a hole is formed in the front surface of a cabinet 1, and a dynamic speaker unit 3 having a diaphragm 2 is mounted over the hole. A resonance port (bass-reflex port) 8 having a sound path 7 is arranged below the speaker unit 3. The resonance port 8 and the cabinet 1 constitute a Helmholtz resonator. In this Helmholtz resonator, an air resonance phenomenon occurs due to an air spring in the cabinet 1 as a closed cavity and an air mass in the south path 7. A resonance frequency f.sub.op is given by: EQU f.sub.op =c(A/lV).sup.1/2 /2.pi. (3)
where c is the velocity of sound, A is the sectional area of the sound path 7, l is the length of the sound path 7, and V is the volume of the cabinet 1. In a conventional bass-reflex type speaker system according to a standard setting, such a resonance frequency f.sub.op is set to be slightly lower than the lowest resonance frequency f.sub.oc.sup.' (.apprxeq.f.sub.oc) of the speaker unit 3 which is assembled in the bass-reflex type cabinet 1. At a frequency higher than the resonance frequency f.sub.op, the sound pressure from the rear surface of the diaphragm 2 inverts its phase oppositely in the sound path 7, whereby the direction radiation sound form the front surface of the diaphragm 2 and the sound from the resonance port 8 are in-phase in front of the cabinet 1, thus constituting an in-phase addition to increase the sound pressure. As a result of the in-phase addition, the lowest resonance frequency of the whole system is lowered to the resonance frequency f.sub.op of the resonator. According to an optimally designed bass-reflex type speaker system, the frequency characteristics of an output sound pressure can be expanded even to below the lower resonance frequency f.sub.oc.sup.' of the speaker unit 3. As indicated by an alternate one long and two short dashed line in FIG. 9, the uniform reproduction range can be widened as compare to those of an infinite plane baffle (indicated by a solid line) and a closed type baffle (indicated by an alternate one long and one short dashed line).
In the equations (1) and (2), the equivalent stiffness S.sub.c is inversely proportional to a volume V of the cabinet 1. Therefore, when the speaker system shown in FIG. 7 or 8 is constant-voltage driven, its frequency characteristics, in particular, low-frequency characteristics are influenced by the volume V o the cabinet 1. Thus, it is difficult to make the cabinet 1 and the speaker system compact without impairing the low-frequency characteristics.
For example, in order to compensate for bass sound reproduction capacity reduced when the cabinet is made compact, as shown in FIGS. 10(a) to 10(d), a bass sound may be boosted by a tone control, a graphic equalizer, a special-purpose equalizer, or the like. In this method, an input voltage is increased with respect to a range below f.sub.oc which is not easy to reproduce, thereby increasing a sound pressure. With this method, it is possible to increase a sound pressure below f.sub.oc. However, adverse influences caused by high Q.sub.oc due to a compact cabinet, such as poor transient response at f.sub.oc due to high Q.sub.oc, an abrupt phase shift at f.sub.oc due to high Q.sub.oc, and the like, cannot be eliminated. Therefore, this method merely has an effect of increasing a sound pressure of a bass sound, and cannot provide sound quality equivalent to that of a speaker system which uses a cabinet having an optimal volume V and has proper f.sub.oc and Q.sub.oc.
Furthermore, in the bass-reflex speaker system shown in FIG. 8, in order to flatten frequency characteristics upon constant-voltage driving, for example, the Q value Q.sub.oc.sup.' of the speaker unit 3 assembled in the bass-reflex cabinet must be set to be Q.sub.oc.sup.' =1/.sqroot.3, and the resonance frequency f.sub.oc.sup.' must be set to be f.sub.op =f.sub.oc.sup.' /.sqroot.2, so that the characteristics values (f.sub.o and Q.sub.o) of the speaker unit 3, the volume V of the cabinet 1, and dimensions (A and l) of a resonance port 8, and the like must be matched with high precision, resulting in many design limitations. In this case, Q.sub.oc.sup.' and f.sub.oc.sup.' can be approximated to Q.sub.oc and f.sub.oc in equation (1) and (2).
FIG. 11 shows a negative impedance generator described in U.S. patent application Ser. No. 07/286,869, now U.S. Pat. No. 4,987,564, previously assigned to the same assignee as in the present application.
According to a driving system using such a negative impedance generator (to be referred to as negative resistance driving system hereinafter) as a driver for a speaker system, for causing an output impedance to include a negative resistance -R.sub.0 to reduce or eliminate the voice coil resistance R.sub.V of a speaker, the Q.sub.oc and Q.sub.oc.sup.' can be decreased and Q.sub.op can be increased as compared to those when the speaker is constant-voltage driven by the power amplifier having an output impedance of zero. Thus, the speaker system can be rendered compact, and acoustic output characteristics can be improved.
Such a speaker system using such a negative resistance driving system is described in U.S. patent application Ser. No. 07/323,680, now U.S. Pat. No. 4,997,564 previously assigned to the same assignee as in the present application.
In the above U.S. application, a closed type speaker system or bass-reflex type speaker system to be used in the normal way, that is, subjected to constant-voltage driving, is provided on or in the cabinet thereof with an open duct port having a predetermined size and driven by an amplifier of a negative resistance driving type, thereby expanding low range reproduction characteristics as compared with the original speaker system which was used in said normal way.
However, when the negative resistance driving system of the U.S. Pat. No. 4,987,564 is to be adopted, a special-purpose driving apparatus is necessary, and a conventional power amplifier cannot be used, resulting in a large economical load on a user.